Optical, magnetic and magneto-optic media are primary sources of high performance storage technology, which enable high storage capacity coupled with a reasonable price per megabyte of data stored. The use of optical media has become widespread in audio, video, and computer data storage applications in such formats as compact disc (CD), digital versatile disc (DVD, including multi-layer structures like DVD-5, DVD-9 and multi-sided formats such as DVD-10, and DVD-18), magneto-optical disc (MO), and other write-once and re-writable formats such as CD-R, CD-RW, DVD-R, DVD-RW, DVD+RW, DVD-RAM, and the like, hereinafter collectively “data storage media”. In these formats, data are encoded onto a substrate into a digital data series. In pre-recorded optical media, such as CD, the data are typically pits and grooves embossed on the surface of a plastic substrate using a method such as injection molding, stamping or the like.
In recordable media, the data are encoded by laser, which illuminates an active data layer that undergoes a phase change, thus producing a series of highly reflecting or non-reflecting regions making up the data stream. In these formats, a laser beam first travels through a plastic substrate before reaching the data layer. At the data layer, the beam is either reflected or not, in accordance with the encoded data. The laser light then travels back through the plastic and into an optical detector system where the data are interpreted.
In some applications, it is desirable to have a limited life for an optical disc. For example, sample computer programs are provided to potential customers in order to entice them to purchase the software. The programs are intended to be used for a limited period of time. Additionally, music, movies, and other forms of digital entertainment are currently rented for a limited time period. In each of these applications and others, when that time has expired, the disc must be returned. A need exists for machine-readable optical discs that do not need to be returned at the end of a rental period. Limited-play discs provide a solution to this problem.
Several approaches have been proposed to make a limited play optical disc based on a layer that changes from a non-interfering state (transparent) where it does not interfere with the reliable reading of the information on the optical disc, via an interrogating beam of light, to an interfering state (opaque) where the layer interferes with the optical reading of the data on the disc. The interference with the reading light source may be due to the layer becoming dark, reflective, highly birefringent, pitting, corroding, bending, changing refractive properties or any combination of these. (See for example, U.S. Pat. No. 6,011,772 and U.S. Pat. No. 5,815,484 and herein incorporated by reference in their entirety). It should be pointed out that it is not essential in all applications that the interfering layer cover an entire surface of the disc. It may be desirable to inhibit only the reading of areas containing critical information content.